In numerous industries, residues or solid particles present in a liquid are being separated from the liquid more and more often by filtering rather than by sedimentation which requires voluminous installations and excessively long settling times, and rather than by centrifuging which does not achieve the desired degree of separation and which is made expensive by the necessary energy consumption and maintenance.
Although filtering is based on a simple principle, namely causing a liquid to pass through a filter medium having orifices which allow the liquid to pass but which prevent the particles from passing, many difficulties are encountered in practice.
Firstly, the resistance to liquid flow is due not only to the filter medium itself but also to the layer of particles which accumulate thereon (commonly referred to as filter "cake"), and this requires a certain pressure to be applied to the liquid by means of pumps which consume increasing amounts of energy as the required pressure increases. Naturally the cake is removed periodically once it has reached a certain thickness, so as to ensure that the resistance due to the cake has an acceptable average value.
The cake may be removed by reversing the flow of the liquid either under pressure or merely under gravity, with a portion of the filtered liquid or "filtrate" passing back through the filter medium, thereby loosening the cake which falls under its own weight to the bottom of the filter-containing vat, from which it is evacuated.
This method of proceeding requires a portion of liquid which has already been filtered to be filtered again and this may constitute a considerable expense. It may also be observed that when the accumulated solid material is to be removed, it is accompanied under these circumstances by a considerable quantity of liquid which needs to be removed subsequently, thereby increasing the cost of the process overall.
Naturally, while the cake is being removed, filtering is interrupted for a period of time which varies depending on the difficulties encountered. Although the major portion of the cake comes loose without much difficulty, zones or plates of cake which are relatively firmly attached to the filter medium may remain so that back-washing must be continued for long enough to loosen these difficult plates. Although they can be removed by auxiliary methods, such as emptying the vat and spraying a liquid jet onto the cake, or shaking the filter surfaces, or back-washing at high pressure, such methods are relatively "violent" and may damage the separating power of the filter medium by applying stresses thereto and by damaging its surface layer, and in any event they require rather long periods of interruption.
When returning to filtering after back-washing, even if relatively "gentle" methods have been used for removing the cake, the initial resistance to liquid flow is very low and the liquid consequently passes through the medium very fast, and as a result it undesirably entrains fine solid particles through the filter medium. The initial filtrate must therefore be recycled, and this must also be taken into account in determining the total time for which the filtering activity of an installation is interrupted.
It follows from above that the real time during which a filter installation is effectively operating as a filter is considerably less than the total time it is in operation.
It may also be observed that filter installations are particularly compact because it is possible to juxtapose a very large number of filter medium surfaces in a common filter vat. However, such a compact structure is not without its own drawbacks. The spaces between the filter surfaces are very narrow and under some unexpected or accidental circumstances solid material may accumulate therebetween and form bridges which push against facing filter surfaces. This may damage the filter surfaces and it is also practically impossible to remove a bridge-forming cake of solid material without interrupting operation of the installation and removing it manually. Such bridges are bound to occur sooner or later.
One particular "unexpected" circumstance worth mentioning here is that it may be necessary to filter liquids in which the concentration of solid material is likely to vary unexpectedly and to give rise, accidentally, to such bridges.